LM48413

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SNAS460B – NOVEMBER 2008 – REVISED MARCH 2013

APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM48413 stereo Class D audio power amplifier features a filterless modulation scheme that reduces external

component count, conserving board space and reducing system cost. The outputs of the device transition from

cycle, in phase, causing the two outputs to cancel. This cancellation results in no net voltage across the speaker,

thus there is no current to the load in the idle state.

When an input signal is applied, the duty cycle (pulse width) of the LM48413 output's change. For increasing

output voltage, the duty cycle of one side of each output increases, while the duty cycle of the other side of each

output decreases. For decreasing output voltages, the converse occurs. The difference between the two pulse

widths yields the differential output voltage.

SHUTDOWN FUNCTION

The LM48413 features a low current shutdown mode. Set SD = GND to disable the amplifier and reduce supply

current to 0.03

μA.

value. The LM48413 shutdown input has an internal 300k

Ω pull-down resistor. The purpose of this resistor is to

eliminate any unwanted state changes when this pin is floating. To minimize shutdown current, it should be

driven to GND or left floating. If it is not driven to GND, or floating, a small increase in shutdown supply current

will be noticed.

SPREAD SPECTRUM

The LM48413 outputs are modulated in spread spectrum scheme eliminating the need for output filters, ferrite

beads or chokes. During its operation, the switching frequency varies randomly by 30% about a 390kHz center

frequency, reducing the wideband spectral content and improving EMI emissions radiated by the speaker and

associated cables and traces. A fixed frequency class D exhibits large amounts of spectral energy at multiples of

the switching frequency. The spread spectrum architecture of the LM48413 spreads the same energy over a

larger bandwidth. The cycle-to-cycle variation of the switching period does not affect the audio reproduction,

efficiency, or PSRR.

ENHANCED EMISSIONS SUPPRESSION SYSTEM (E

quality audio reproduction and efficiency. The advanced edge rate control (ERC) embedded within the E2S

system works simultaneously with the spread spectrum already activated. The LM48413 ERC greatly reduces

the high frequency components of the output square waves by controlling the output rise and fall times, slowing

the transitions to reduce RF emissions, while maximizing THD+N and efficiency performance.

DIFFERENTIAL AMPLIFIER EXPLANATION

As logic supplies continue to shrink, system designers are increasingly turning to differential analog signal

handling to preserve signal to noise ratios with restricted voltage swings. The LM48413 features two fully

differential speaker amplifiers. A differential amplifier amplifies the difference between the two input signals.

Traditional audio power amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction of

SNR relative to differential inputs. The LM48413 also offers the possibility of DC input coupling which eliminates

the input coupling capacitors. A major benefit of the fully differential amplifier is the improved common mode

rejection ratio (CMRR) over single-ended input amplifiers. The increased CMRR of the differential amplifier

reduces sensitivity to ground offset related noise injection, especially important in noisy systems.

POWER DISSIPATION AND EFFICIENCY

The major benefit of a Class D amplifier is increased efficiency versus a Class AB. The efficiency of the

LM48413 is attributed to the region of operation of the transistors in the output stage. The Class D output stage

acts as current steering switches, consuming negligible amounts of power compared to a Class AB amplifier.

Most of the power loss associated with the output stage is due to the IR loss of the MOSFET on-resistance,

along with switching losses due to gate charge.

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